My Mini Schnauzer has really bad separation anxiety. What more can I do?

I have a 7 month old Mini Schnauzer. He's always had really bad separation anxiety since day one and he's only got slightly better recently. I know I'm to blame as when we first got him, I was at home studying a lot and we never really left him for more than 2 hours max. In those 2 hours, he would howl loudly throughout the entire time (we record him every time we leave) and for that reason, I would try my best not to leave him at all (which obviously hasn't helped)! Now we're at a point where I need to be able to leave him. I have a 4 hour a day job (which I got on purpose so I wouldn't have to leave him for too long). This is what we have tried in order to help his anxiety: aromatherapy diffuser, TV/music, Kong's, putting our coats on/picking up keys then sitting down or taking them off/putting them down again etc (we did this over a few days but didn't seem to help), we've tried him in his crate/out of his crate, we don't make a fuss when we leave or return, we've even watched him on skype while we sit in the car and make distracting noises every time he's about to howl. This is our current routine which seems so minimise his howling: I take him for a run in the fields to wear him out a bit, then get ready for work before siting in the living room, I then exchange his toys for some toys he hasn't played with for a while, put some classical music on, sit with him for a while longer (without fussing with him) then quietly leave. Most of the time he howls so loudly that he loses his voice! I don't know what else I can do, this is ruining my life!!!!